An MRI device is a device for measuring NMR signals (echo signals) generated by atomic nucleus spins constituting an examinee, particularly a tissue of a human body, and two-dimensionally or three-dimensionally imaging the shape or function of a cephalic part, an abdominal part, four limbs or the like of the examinee. In the imaging operation, echo signals are added with different phase encodes by gradient magnetic field, subjected to frequency encoding and measured as time-series data. The measured echo signals are subjected to two-dimensional or three-dimensional Fourier Transform to be reconstructed into an image.
There is an imaging function called as MR angiography (hereinafter abbreviated to MRA) for depicting a blood vessel by using this MRI device. The MRA imaging function includes a phase contrast MRA (hereinafter referred to as PC-MRA) method for imaging blood in a blood vessel by using the principle that transverse magnetization phase of blood is shifted in accordance with blood stream rate. According to the PC-MRA method, the complex difference is taken between an image acquired by applying a positive-polarity flow encode pulse and an image acquired by applying a negative-polarity flow encode pulse, thereby obtaining a blood vessel image reflecting a flow velocity value.
According to the PC-MRA method described above, only one type of flow encode which is optimally matched with the average flow velocity of a blood vessel as an imaging target is normally set to depict the target blood vessel. For example, when a cervical part area is subjected to blood vessel imaging by the PC-MRA method, the cervical part area contains right and left carotid arteries, right and left arteria vertebralis, arteria basilaris, etc. as main blood vessels, and the flow encode is set in conformity with the average flow velocity of 40 cm/s of the carotid arteries as large blood vessels of the cervical part.
However, in general, plural blood vessels having average velocities which are different from that of a blood vessel as an imaging target exist in an area to be imaged. Therefore, in the PC-MRA method using only one type of flow encode, the target blood vessel is depicted with high brightness, however, the other blood vessels are depicted with low brightness, so that all the blood vessels different in blood stream rate do not become blood vessel images which are depicted with high brightness.
Therefore, (patent document 1) discloses a blood vessel imaging technique that the flow encode is set to a relatively small flow encode when data of low frequency components at a phase encode step are collected and set to a relatively large flow encode when data of high frequency components at the phase encode step are collected, whereby both of a main blood vessel which has a relatively high flow velocity and is large in diameter and a peripheral blood vessel which has a relatively low flow velocity and is small in diameter can be excellently depicted.